Methods of dehydrating biomass-derived carbohydrate are important and attractive chemical reactions because they produce valuable anhydrosugar compounds from renewable sources. Anhydrosugars are sugar derivatives that formally arise by the elimination of one or more water molecules from arbitrary hydroxyl groups of the parent aldose or ketose derived from carbohydrates. They usually contain a bicyclic or tricyclic skeleton composed of oxirane, oxetane, oxolane (tetrahydrofuran), and oxane (tetrahydropyran) rings (e.g., levoglucosan, 1,6-anhydro-β-d-glucofuranose, levoglucosenone, 1,4:3,6-dianhydro-α-D-glucopyranose, mannosan, galactosan, etc.). They can also be compounds without bicyclic or tricyclic skeletons, such as 5-hydroxymethylfurfural, furfural, maltol, etc. Examples of anhydrosugars are shown in FIG. 1.
Carbohydrates, with pentose and hexose as the building blocks, include monosaccharides (e.g., pentose and hexose, and derivatives thereof), disaccharides (e.g., sucrose, maltose, lactose, cellobiose, and derivatives or byproducts thereof), and polysaccharides (e.g., maltodextrins, starches, cellulose, and derivatives or byproducts thereof). Any of these can be used as starting materials to make desired anhydrosugars.
For pentose sugars (C5H10O5, e.g., arabinose, xylose, ribose, and lyxose), the corresponding dehydration products will have one of the following formulas: C5H8O4, C5H6O3, C5H4O2, or C5H2O. For hexose (C6H12O6, e.g., glucose, fructose, mannose, and galactose), the corresponding dehydration products will have one of the following formulas: C6H10O5, C6H8O4, C6H6O3, or C6H4O2.
Levoglucosan is one well-known anhydrosugar with uses as a raw material for production of pharmaceuticals (e.g., Avermectin), polymers and surfactants. For example, levoglucosan can be used to create SGLT2 inhibitors used in generic diabetes drugs. However, large-scale production of levoglucosan remains elusive due to the extensive purification steps required (Czernik and Bridgwater 2004). It has been known for many years that starch containing materials and cellulose, or lignocellulosic materials (e.g., wood), may be converted to levoglucosan by pyrolysis. But most researchers have identified the efficient isolation of levoglucosan from the pyrolytic liquids as the main difficulty in the efficient production of levoglucosan (Czernik and Bridgwater 2004; Radlein 2002).
If a simple, cheap and reliable method could be found for levoglucosan production, it would be an important breakthrough in this field. Thus, several processes have been patented for generation, recovery and purification of levoglucosan from pyrolysis of cellulose or lignocellulose.
In U.S. Pat. No. 1,437,615, for example, a batch vacuum pyrolysis process was described (pyrolysis temperature range is 200° C. to 300° C., pressure is 12-15 mm Hg). The starting substrate was cellulose, starch and sawdust. The pyrolysis products derived from carbohydrate materials were fractionally condensed, dehydrated and extracted into boiling acetone to get the crystallized levoglucosan. However, this “acetone method” used a volatile and flammable solvent.
In U.S. Pat. No. 3,309,356, azeotropic distillation and methyl isobutyl ketone extraction were combined to separate levoglucosan from pyrolyzed Douglas fir sawdust (pyrolysis temperature: 600-1500° F.). The crude products included: char, light gases, and condensable gases (e.g., tars, substituted phenolic materials, levoglucosan and carbohydrate derived acids).
U.S. Pat. No. 3,374,222 used the same substrate, pyrolysis condition and pyrolysis crude products as U.S. Pat. No. 3,309,356. Impurities, such as polymeric carbohydrate-derived acids, were precipitated by adding basic metal salts (e.g., alkaline earths, aluminum, lead or zinc). Levoglucosan was then crystallized from the concentrated filtrate.
In U.S. Pat. No. 3,235,541, a wood pulp containing 96% alpha-cellulose was used as the substrate. The pyrolysis temperature was 350° C., 400° C. and 450° C. The crude products included char, light gases and crude levoglucosan. This patent describes a process that used chloroform to remove the colored impurities in cellulose-derived pyrolysis oil in order to get high purity levoglucosan. However, chloroform is highly toxic.
In U.S. Pat. No. 5,395,455, partial oxidative pyrolysis processes were conducted using pyrolysis temperatures between 400° C. and 650° C. The substrate was either (1) commercial cellulose washed by 5% sulfuric acid, or (2) hybrid poplar wood which had been pre-hydrolyzed using 5% sulfuric acid. The pyrolysis crude products contained 35-51.4% anhydrosugars (e.g., levoglucosan, anhydroglucofuranose and cellobiosan). The aqueous phase from water induced phase separation of the pyrolysis products was first dewatered, then extracted by hot alcohol followed by charcoal treatment, then after solvent evaporation, the levoglucosan remained.
In U.S. Pat. No. 5,023,330, the pyrolysis products were condensed and neutralized with alkali to a pH of about 6 to 8. Then a chromatographic method was used to purify levoglucosan. The substrate was commercial starch and wheat flour, and the pyrolysis temperature was 200° C. to 500° C.
In U.S. Pat. Nos. 5,371,212 and 5,432,276, cellulose/starch/waste newsprint were washed with hot acid and pyrolyzed at a temperature of 350-375° C. The pyrolysis liquid was extracted with methyl isobutyl ketone, neutralized with a base, freeze-dried and extracted by ethyl acetate. The inventors claimed the method offered a way to obtain a highly pure crystalline form of levogucosan. However, this process seems to be very complex and is unlikely to be cost effective.
Thus, although some progress has been made, what is needed in the art are better, more robust methods with even better outcomes. Ideally, the method would be simple, use readily available inexpensive, and preferably non-toxic, ingredients, and produce a high purity of the desired product, thus obviating the need for extensive purification.